1. Field of the Invention
The invention relates to scanners and more particularly to digital scanners for detecting changes in the gray scale of a scanned image to generate a line drawing with the position of the lines corresponding to the central portion of the changes in the gray scale of the scanned image.
2. Description of the Prior Art
Typical prior art systems for scanning an image to generate a line drawing with the position of the lines corresponding to the changes in the gray scale of a scanned image have relied on analog techniques. Changes in the gray scale were generally detected by taking either the first or second derivative of the video signal from the scanner used to scan the image, or some combination of these, to generate signals indicative of the changes in gray scale of the scanned image. A typical example of such a system is disclosed in Ser. No. 585,641 now U.S. Pat. No. 4,072,218 assigned to the same assignee as this application. These systems had limited capability of detecting changes in the gray scale which were parallel to the direction of scan. Additionally the line width of the line drawing generated by these systems tended to vary depending on the direction of the line with respect to the direction of scanning. The true edges of out-of-focus objects were usually displaced because of the gradual changes in the gray scale resulting from such edges.